Device for the successive amplification of a number of low voltages



Oct. 19, 1965 e. KLEIN ETAL 3,213,290

DEVICE FOR THE SUCCESSIVE AMPLIFICATION OF A NUMBER OF LOW VOLTAGES Filed Oct. 8, 1959 2 Sheets-Sheet l LIMITER CIRCUIT --IS I E INPUTS /D|STRIBUTOR 2Q; PULSE SHAPER 21 AND THRESHOLD TRIG GER INVENTOR gerrlt kleln ohannes JOCObLIS zuclberg van zelst BY x AGxT

Oct. 19, 1965 G. KLEIN ETAL 3,213,290

DEVICE FOR THE SUCCESSIVE AMPLIFICATION OF A NUMBER OF LOW VOLTAGES Filed Oct. 8, 1959 2 Sheets-Sheet 2 INVENTOR gerrihklein johcnncs jGCObUS zcclberg van BY zelst AGEN United States Patent 3,213,290 DEVICE FOR THE SUCCESSIVE AMPLTFICATIGN OF A NUMBER OF LOW VOLTAGES Gerrit Klein and Johannes Jacobus Zaalberg van Zelst, Eindhoven, Netherlands, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Oct. 8, 1959, Ser. No. 845,140 Claims priority, application Netherlands, Oct. 31, 1958, 232,823 4 Claims. (Cl. 30788.5)

The present invention relates to devices for the successive amplification of a number of low voltages by means of a corresponding number of pro-amplifiers having separate input circuits and a common output circuit, in which the pre-amplifiers are successively and periodically rendered operative under control of a distributor. Such devices, comprising tube pre-amplifiers and a common tube amplifier, are known, for example from US. patent specification 2,666,868, in which the preamplifiers are provided with pentodes and are controlled by a distributor made up of bistable triggers having tubes which supply positive voltage pulses to the third grids of the pentodes of the pre-arnplifiers. However, it is also known to control such pro-amplifiers by means of positive or negative voltage pulses applied to the anode, to the screen grid, to a field grid or to the cathode of the preamplifier tube.

From Dutch Patent No. 61,742, October 15, 1948, it is also known to use pushpull pre-arnplifiers, the two tubes of which are coupled together by means of a common cathode resistor in order to reduce the voltage pulses occurring when rendering one of them inoperative and the other operative. Each of the pro-amplifiers is in turn cut off by a positive voltage supplied to the cathodes of its two tubes; the positive voltage is supplied through the cathode-anode path of a tube of a multivibrator.

The object of the present invention is to provide a device of the same type, in which push-pull transistor pro-amplifiers are used and the suitable switching properties of transistors are employed to advantage in order to render the pre-amplifiers alternatively operative.

The device according to the invention is characterized in that each of the pre-amplifiers comprises a conventicnal amplification stage having two transistors of the same type, between the base-electrodes of which the corresponding low voltage is applied; the emitter-circuits of the pre-amplifier transistors comprise a common resistor ot comparatively high value with respect to the internal emitter resistance of each transistor; the collector-electrodes of the two transistors are connected to a source of reverse voltage via a symmetrical load circuit and are symmetrically coupled to said common output circuit and to the input terminals of an additional common amplifier; the distributor comprises a number of switch transistors corresponding to the number of voltages to be amplified, which are successively and periodically rendered operative; through the emitter-collector paths of the switch transistors the respective common emitter-circuits of the pre-amplifier stages are successively and periodically connected to a supply of forward voltage.

In order to make the common amplifier insensitive with respect to the voltages produced when switching the preamplifier stages on and off since these voltages, although strongly attenuated, still reach its input terminals, this common amplifier preferably comprises a controlled stage which is periodically rendered operative only after switching on any of the pre-amplifier stages and rendered inoperative again before switching off the same pre-amplifier stage.

In order that the invention may be readily carried into effect, it will now be described in greater detail with reference to the accompanying drawings, in which:

FIG. 1 is the circuit diagram of one embodiment of the device according to the invention,

FIG. 2 is a time diagram of the currents through the common emitter circuits of the various pie-amplifier stages of this device,

FIG. 3 shows voltage-time diagrams at various points of the common amplifier of this device and of a circuit arrangement for controlling a stage of this amplifier, and

FIG. 4 shows a conventional voltage-scanning arrangement.

The device shown in FIG. 1 comprises a number of pairs of input terminals 1, 2 11. Each pair of input terminals is connected between the base electrodes of two transistors 3 and 4 of a pre-amplifier stage. The emitter electrodes of the transistors of each pre-amplifier stage are connected together through a potentiometer 5, the variable tap of which is connected to a common emitter resistor 6 and, via this resistor, to the collector elec trode of a third transistor 7 of the same conductivity type. The transistor '7 is a switch transistor which forms part of a distributor. Each of the various transistors 7 becomes conductive in turn, for example for approximately 30 sec., so that the corresponding pre-amplifier stage becomes operative during this period, owing to its emitter circuit being closed via the transistor 7 and a low-value common resistor 8. The resistor 8 lies between the emitter electrodes of all the transistors 7 and the positive terminal +E of a supply of, for example, 16.5 v. The collector electrodes of all the transistors 3 are collectively connected to one half of a symmetrical load circuit and the collector electrodes of all the transistors 4 are likewise connected to a common other half of this load circuit. Each half of the common circuit comprises the emitter-collector path of a transistor 9 and 10 respectively of the same conductivity type as the transistors 3 and 4 of the pre-amplifier stages and as the switch transistors 7, in series-combination with a load resistor 11 and 12 respectively. The common point of the resistors 11 and 12 (the centre tap of the load circuit) is connected to the negative terminal E of the supply via a common load resistor 13. The base electrodes of the transistors 9 and 10 are connected to a point of constant potential of, for example, 7.5 v. with respect to ground via a common resistor 14 and the collector electrodes of these two transistors are directly connected to the input terminals 15 of a common wide-band direct current amplifier 16 with output terminals 17. This amplifier comprises a push-pull input. A practicable wide-band direct current amplifier with tubes as amplifying elements had, for example, a flat transmission characteristic up to a frequency of 2 mc./s. and an overall amplification factor of 8000 times with a rejection factor of 5000, and even of 100,000 with adjustment of the input stage, with respect to cophase potential variations at the two input terminals.

The potential with respect to ground at the terminal +E and the potential of 7.5 v. applied to the base electrodes of the transistors 9 and 10 are obtained by means of a potentiometer connected between the terminals E and +E of the supply and comprising resistors 22, 23, 24 and 25, the resistors 23, 24 and 25 being shunted by electrolytic capacitors 26, 27 and 28 respectively.

The distributor with the transistors 7 contains additional elements by which these transistors are intercoupled in a kind of ring-counter, so that each becomes conductive at the instant the preceding one is cut oif and that each remains conductive only for a short time, for example during said period of approximately 30 ,usec.

This is brought about by coupling a point of the collector circuit of the last switch transistor via a capacitor 29, or electrode of the next following switch transistor via a capacitor 29. Finally, the base electrode of the first switch transistor 7 is coupled to a point of the collector circuit of the last switch transistor via a capacitor 29, or to a supply of negative control of clock-pulses 21 via a switch 20. The base electrode of each of the transistors 7 is coupled to the terminal +E through a resistor 30 and a parallel-connected diode 31 connected in the passdirection with respect to the base current of the corresponding transistor 7.

In order to be able to interrupt completely and suddenly the current through the resistors 6, the capacitors 29 are connected to a tapping of a separate load circuit 48, 49 of the preceding transistor 7.

When a transistor 7 is cut off, a negative pulse is supplied to the base electrode of the following transistor 7 via the corresponding capacitor 29 and this transistor remains conductive until this capacitor has become sufficiently charged via the resistors 30 and 49 and until its base electrode has again become positive with respect to its emitter. The diode 31 suppresses the positive-going pulses at the beginning of each conductivity period of a transistor 7 and maintains the base electrode of the following transistor 7 and the corresponding electrode of the capacitor 29 at the potential of the terminal l-E during this period.

Each of the pre-amplifier stages I, II N is rendered operative in its turn by the distributor with the switch transistors 7. Each of these stages operates as a symmetrical push-pull stage during the short time the corresponding transistor 7 is conductive. The resistor 6 has a high value with respect to the emitter resistance of each of the transistors 3 and 4, so that a satisfactory balance is obtained by coupling across this resistor. By adjusting the potentiometer 5, this balance can still be improved, if necessary, as a result of which differences between the respective leakage currents I and/or between the respective base-collector-current amplification factors of the transistors 3 and 4 can be compensated. Even if the voltages between the input terminals 1, 2 or n of this pre-amplifier stage are asymmetrical with respect to ground, a substantially symmetrical voltage is produced at the collector electrode of this pre-amplifier stage. The parallel-connected collectors of the transistors 3 and 4 respectively constitute a non-negligible capacity, so that it is desirable to connect these collector electrodes to a load circuit having low impedances. These impedances are constituted by the halves of the symmetrical input circuits of the transistors 9 and 10 in common base arrangement, which act as matching elements.

The amplified voltages produced across the resistors 11 and 12 successively correspond to the various voltages applied between the input terminals 1, 2 n. However, an asymmetrical component is inevitably superimposed on these symmetrical voltages, owing to the fact that the change-over from one pre-amplifier stage to the next following takes some time, albeit a very short one. Consequently, between the various periods during which the various pre-amplifier stages are successively operative, there exist very short periods during which no pre-amplifier stage is operative or during which one pre-amplifier stage is not yet fully inoperative and the next-following is not yet fully operative. During these intermediate periods, the voltage at the collector electrodes of both transistors 9 and 10 would thus be substantially equal to that of the negative terminal E. Hence, the common amplifier 16 is controlled by a voltage which reaches its two input terminals 15 in phase opposition and by an interference voltage consisting of short peaks which simultaneously drive these two base electrodes comparatively strongly in the negative direction.

In order to prevent overdriving of the first stage of the wide-band direct current amplifier by these negative voltage peaks, the latter are limited by means of two diodes 18 and 19 respectively and/or at least partially suppressed or compensated by voltage peaks of opposite polarity produced by a transistor 32, the collector of which is connected to the common point of the resistors 11, 12 and 13 of the common load circuit. The emitter of this transistor lies at a point of constant potential and its base electrode is coupled to the collector of one of the transistors 9 and 10 via a capacitor 33, and polarized in the forward direction via a high-value resistor 34 connected between the collector and the base electrode of this transistor 32. The disturbing voltage peaks of same polarity, produced across each of the two resistors 11 and 12 consequently drive the transistor 32, which operates as a class A amplifier and produces corresponding voltage peaks of opposite polarity across the resistor 13. The final result is that the change-over voltage peaks are strongly suppressed, since the potential of the collectors of transistors 9 and l0apart from the amplified input voltages which alternately appear between themvaries but very slightly.

The time diagram shown in FIG. 2 illustrates the successive operations of the switch transistors 7. The first line shows the current through the resistor 6 of the common emitter-circuit of the transistors 3 and 4 of the pre-amplifier stage N. The second line shows the corresponding current through the emitter circuit of the preamplifier stage I, the third line shows the emitter current of the pre-amplifier stage II, and the last line shows the emitter-current of a pre-amplifier stage NI not shown in FIG. 1.

FIG. 3 shows voltage time diagrams at various points of the circuit arrangement shown in FIG. 1. The first line shows the time diagram of the voltage at the collec tor of one of the transistors 9 and 10. On this part of the diagram, large negative voltage peaks are to be seen, which are suppressed or compensated for example down to a level 0 (dot and dash line), by means of the diodes 18 and 19 and/or of the transistor 32. As may be seen, considerable negative voltage peaks are still transmitted to the common input terminals 15 at each change-over. In order to provide better operating conditions for a measuring or regulating device connected to the output terminals, it would be desirable to amplify only the voltage differences between the collector electrodes of the transistors 9 and 10 and to transmit them to the output terminals 17 during the periods i-u. This is practically effected for example at a higher voltage level, by rendering one of the stages of the wide-band current-amplifier 16 operative only after switching on each pro-amplifier stage, and rendering it inoperative again before switching off the same pre-amplifier stage. By carefully and accurately balancing this common stage, the disturbing peaks are strongly suppressed. Another possibility is to control one of the stages, at a higher voltage level, via a voltagescanning arrangement and only during the intervals i-u. In this manner, only the parts of the voltage curve of the first line of FIG. 3 between the points i and u are transmitted. This is to be understood as if the curve of the first line of FIG. 3 represented the voltage variations at the collector of the transistor 9, another curve of the same character representing the voltage variations at the collector of the transistor 10. The said common amplifier 16 transmits and consequently amplifies periodically and successively, during a part of the operation period of each pre-amplifier stage, only the voltage difference between the input terminals of the pre-amplifier stage operating at this instant. Both possibilities of transient suppression can be realized by means of an amplifying element operating as a switch and controlled by the very small voltage pulses produced by the current pulses across the low-value resistor 8 and shown in FIG. 2. The second line of FIG. 3 shows these successive voltage pulses, each of them naturally having a leading edge and a trailing edge of limited slopes. These approximately square negative pulses closely following each other are transmitted to the base electrode of a transistor 35, via an integrating and a differentiating network comprising a series-resistor 36, a shunt-capacitor 37, a series-capacitor 38 of comparatively low value, and a shunt-resistor 39. By integrating network 36, 37, the pulses across the resistor 8 are deformed, approximately as shown on an exaggerated scale on the third line of FIG. 3. By the differentiating network 38, 39, these pulses are again further deformed approximately as shown in the last line of FIG. 3, where the dot and dash line d shows the amplification threshold of the transistor 35. It may be seen that this transistor becomes conductive only during a part of each of the pulses of the second line of FIG. 3 and produces amplified positive pulses across its collector resistor 40. These pulses drive an additional amplifier transistor 41, the base electrode of which is coupled to the collector electrode of the transistor 35 via a capacitor 42, and the emitter electrode of which lies at the +13- terminal. The base of this transistor is polarized in the forward direction by means of a potentiometer having resistors 43 and 44, and its collector lies at the E-terminal via a load resistor 45. This transistor thus amplifies the signal produced across the resistor 40 in the collector circuit of the transistor 35 and comprising positive pulses corresponding to the part of the signal at the base of the transistor 35 below the dot and dash line d of the last line of FIG. 3. Negative, approximately square pulses corresponding to the same parts of the signal at the base of the transistor 35, are thus produced across the resistor 45, and these negative pulses are applied, via a coupling capacitor 46, either to said one stage of the common amplifier 16 to render this stage operative only during said negative pulses, or they are applied to said voltage-scanning arrangement. The stage of the common amplifier controlled by the negative pulses includes, for example, two push-pull connected tubes and a third tube operating as a switch, with the cathode-anode path of the third tube in the common cathode circuit of the push-pull amplifier stage. An example of such a circuit arrangement including three tubes is shown in FIG. 1 or FIG. 2 of British Patent 685,358, December 31, 1952.

FIG. 4 shows an example of a voltage-scanning arrangement. This arrangement comprises a transformer with a primary winding 51 to which the square control current pulses are supplied via the resistor 45 of FIG. 1. This transformer has two equal secondary windings 52 and 53, with centre tappings for connection to the output terminals of a stage of the common amplifier 16, preferably a stage of low-internal resistance, for example a push-pull stage of cathode followers. Diodes 54, 54', 55, 55, each of them connected in series with the parallelcombination of a resistor 56 and of a capacitor 57, 56 and 57', 58 and 59 and 58 and 59', respectively, are connected to the ends of the windings 52 and 53. On the other hand, the diodes 54 and 54 and 55 and 55 respectively can also be connected to the two input terminals 61 of the following amplifier stage of the common amplifier 16 and are furthermore connected to a point P of constant potential via capacitors 63 and 64 respectively.

During each of the control current pulses, all the diodes are rendered conductive, so that each of the capacitors 63 and 64 is charged or discharged to the potential of the corresponding input terminal 60. Between the control-current pulses, all the diodes are cut off by the detected voltage produced by the control current pulse across the corresponding series-capacitor 57 or 57' and 59 or 59' respectively. At the beginning of the next-following control-current pulse, these capacitors have become largely discharged via the corresponding resistor 56 or 56' and 58 or 58 respectively.

The following amplifier stage thus receives input voltage only for the duration of the control-current pulses; this is an integrated input voltage. The amplifier stage is thus rendered insensitive to any transient voltage pro- 6 duced by the switching from any of the pre-amplifier stages I N to the next following one.

Differences between the diodes of each pair can be compensated by means of an additional variable series-re sistor and/or by means of a low-value variable parallel capacitor, and any switching voltage peaks still transmitted are suppressed by the integrating action of the capacitors 63 and 64.

Under particular circumstances and according to the requirements imposed upon the device, several of the more sophisticated improvements explained hereinabove are not strictly necessary for the satisfactory operation of the device. For example, with a comparatively small number of pre-amplifier stages and/ or with a comparatively slow commutation of these stages, the matching transistors 9 and 10 may be omitted, and the collectors of all the transistors 3 and 4 respectively may be connected directly to the load resistors 11 and 12 respectively, as shown in dotted lines on FIG. 1. Alternatively, the limiting diodes 18 and 19 and/or the suppression or compensation transistor 32 with the common load resistor 13 might be omitted under certain circumstances. Furthermore, a controlled stage or a coupling by way of a voltage scanning arrangement in the common directcurrent amplifier is not strictly necessary either.

However, with a large number of pre-amplifier stages and/ or a high switching frequency, all the above improvements are, if not strictly necessary, at least highly desirable for a satisfactory operation of the arrangement, particularly if the input voltages applied to the input terminals .1, 2 n are of comparatively low value, for example of the order of 0.1 mv. or lower. In such cases, it is self-evident that all the interference voltages should be suppressed or compensated as well as possible.

The embodiment described with reference to FIG. 1 is, for example, eminently suitable for the amplification of a large number of thermo-voltages. The input terminals 1, 2 n are connected to a corresponding number of thermo-elements, for example to a thousand different thermo-elements, so as to permit the device to effect the periodic amplification of, say, 1000 different temperature indications at the rate of thirty times a second. These amplified thermo voltages are used, for example, for monitoring and practically uninterruptedly controlling 1000 different temperatures (this could be, for example, 30 times per second), for example at 1000 different points of a nuclear reactor.

It will be obvious to those skilled in the art that the inventive concept herein disclosed can be embodied in apparatus other than that specifically described without departing from the essence of the invention, the scope of which is set forth in the appended claims. It is to be particularly noted that any quantitative values set forth in the specification are given for illustrative purposes only in order to show a preferred means for carrying out the invention. Other quantitative values can be equally suitable for the inventive purpose.

What is claimed is:

1. A device for the successive amplification of a plurality of low voltages by means of a corresponding number of pre-amplifiers and a symmetrical common load and output circuit coupled to the input terminals of a common amplifier, the pre-amplifiers being successively and periodically rendered operative under control of a distributor, each pre-amplifier including a pair of push-pull connected transistors of the same conductivity type, the emitter circuits of which comprise a common resistor of comparatively high value with respect to the internal emitter resistance of each transistor, and the collector electrodes of which are coupled through said symmetrical load circuit to a voltage source reversely biasing said collector electrodes, said distributor comprising a plurality of switch transistors corresponding to the plurality of voltages to be amplified, means for rendering said r switch transistors successively and periodically conductive, the emitter-collector path of each switch transistor being connected between the common emitter circuit of the corresponding pre-amplifier stage and a supply of forward voltage, and a compensating transistor of the same conductivity type as the pre-amplifier transistors, the emitter of the compensating transistor being connected to a point of constant potential, its collector being connected to a center tap of said common load circuit and said supply source through a compensating resistor, and its base being coupled to one of the terminals of the load circuit, whereby voltage peaks of the same polarity set up across the two halves of the symmetrical load circuit when a pre-amplifier is switched oif and the nextfollowing pre-amplifier is switched on are at least partially compensated by the voltage drop produced across said compensating resistor by the corresponding peaks of the collector current of said transistor.

2. A device for the successive amplification of a plurality of low voltages by means of a corresponding number of pre-amplifiers and a symmetrical common load and output circuit coupled to the input terminals of a common amplifier, the pre-amplifiers being successively and periodically rendered operative under control of a distributor, each pre-amplifier including a pair of push-pull connected transistors of the same conductivity type, the emitter circuits of which comprise a common resistor of comparatively high value with respect to the internal emitter resistance of each transistor, and the collector electrodes of which are coupled through said symmetrical load circuit to a voltage source reversely biasing said collector electrodes, said distributor comprising a plurality of switch transistors corresponding to the plurality of voltages to be amplified, means for rendering said switch transistors successively and periodically conductive, the emitter-collector path of each switch transistor being connected between the common emitter circuit of the corresponding pre-amplifier stage and a supply of forward voltage, and pulse-controlled gating means arranged in the signal transmission path in said common amplifier, and pulse forming means controlled by said distributor,

said pulse forming means producing a series of controlpulses including one pulse during each closure of each of said switch-elements of the distributor, each of said pulses beginning slightly after the switching on of a switch transistor and ending slightly before the switching off of the same switch transistor, said pulses controlling the gating means to permit transmission through said signal transmission path, whereby the common amplifier is rendered insensitive with respect to voltages produced at its input terminals when switching the pre-amplifiers on and oif.

3. A device as claimed in claim 2, wherein said pulsecontrolled gating means comprise an amplifier stage of said common amplifier, said amplifier stage having two push-pull connected amplifier elements, and a third amplifier element operating as a switch and having a control electrode coupled to said pulse forming means, a common electrode and an output electrode, the current path between said cornmonand output-electrodes being connected in series in a common supply circuit for the two amplifier-elements of said push-pull amplifier stage;

4. A device as claimed in claim 2, wherein said pulsecontrolled gating means are arranged at a point of said signal-transmission path of said common amplifier where the voltage level of the transmitted signals is higher than at the input of the common amplifier.

References Cited by the Examiner UNITED STATES PATENTS 2,572,792 10/51 White 328-104 2,627,039 1/53 MacWilliams 30788.5 2,662,175 12/53 Staal 179-l5 2,925,491 2/60 Hoeppner et al. 328- 2,944,216 7/60 Allenden 33030 2,967,951 1/61 Brown.

DAVID J. GALVIN, Primary Examiner.

ELI J. SAX, HERMAN KARL SAALBACH, ARTHUR GAUSS, Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,213,290 October 19, 1965 Gerrit Klein et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 36, after "common" insert load column 3, lines 2 and 3, strike out "circuit of the last switch transistor via a capacitor 29, or electrode of the next following switch transistor via a" and insert instead circuit of each of the said switch transistors 7 to the base electrode of the next following switch transistor via a column 6, lines 45 and 46, strike out "1000 different temperatures (this could be, for example, 30 times per second) for example at 1000 different" and insert instead (this could be, for example, 30 times per second) 1000 different temperatures, for example at 1000 different Signed and sealed this 29th day of November 1966.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A DEVICE FOR THE SUCCESSIVE AMPLIFICATION OF A PLURALITY OF LOW VOLTAGES BY MEANS OF A CORRESPONDING NUMBER OF PRE-AMPLIFIERS AND A SYMMETRICAL COMMON LOAD AND OUTPUT CIRCUT COUPLED TO THE INPUT TERMINALS OF A COMMON AMPLIFIER, THE PRE-AMPLIFIERS BEING SUCCESSIVELY AND PERIODICALLY RENDERED OPERATIVE UNDER CONTROL OF A DISTRIBUTOR, EACH PRE-AMPLIFIER INCLUDING A PAIR OF PUSH-PULL CONNECTED TRANSISTORS OF THE SAME CONDUCTIVITY TYPE, THE EMITTER CIRCUITS OF WHICH COMPRISES A COMMON RESISTOR OF COMPARATIVELY HIGH VALUE WITH RESPECT TO THE INTERNAL EMITTER RESISTANCE OF EACH TRANSISTOR, AND THE COLLECTOR ELECTRODES OF WHICH ARE COUPLED THROUGH SAID SYMMETRICAL LOAD CIRCUIT TO A VOLTAGE SOURCE REVERSELY BIASING SAID COLLECTOR ELECTRODES, SAID DISTRIBUTOR COMPRISING A PLURALITY OF SWITCH TRANSISTORS CORRESPONDING TO THE PLURALITY OF VOLTAGES TO BE AMPLIFIED, MEANS FO RENDERING SAID SWITCH TRANSISTORS SUCCESSIVELY AND PERIODICALLY CONDUCTIVE, THE EMITTER-COLLECTOR PATH OF EACH SWITCH TRANSISTOR BEING CONNECTED BETWEEN THE COMMON EMITTER CIRCUIT OF THE CORRESPONDING PRE-AMPLIFIER STATE AND A SUPPLY OF FORWARD VOLTAGE, AND A COMPENSATING TRANSISTOR OF THE SAME CONDUCTIVITY TYPE AS THE PRE-AMPLIFIER TRANSISTORS, THE EMITTER OF THE COMPENSATING TRANSISTOR BEING CONNECTED TO A POINT OF CONSTANT POTENTIAL, ITS COLLECTOR BEING CONNECTED TO A CENTER TAP OF SAID COMMON LOAD CIRCUIT AND SAID SUPPLY SOURCE THROUGH A COMEPNSATING RESISTOR, AND ITS BASE BEING COUPLED TO ONE OF THE TERMINALS OF THE LOAD CIRCUIT, WHEREBY VOLTAGE PEAKS OF THE SAME POLARITY SET UP ACROSS THE TWO HALVES OF THE SYMMETRICAL LOAD CIRCUIT WHEN A PRE-AMPLIFIER IS SWITCHED OFF AND THE NEXTFOLLOWING PRE-AMPLIFIER IS SWITCHED ON ARE AT LEAST PARTIALLY COMPENSATED BY THE VOLTAGE DROP PRODUCED ACROSS SAID COMPENATING RESISTOR BY THE CORRESPONDING PEAKS OF THE COLLECTOR CURRENT OF SAID TRANSISTOR. 